In this study of Pseudomonas aeruginosa, the expression consistency of seven commonly used reference genes (rpoD, proC, rpoS, 16S, algD, gyrA and ampC) was assessed in P. aeruginosa laboratory (PAO1) and clinical (LESB65) isolates grown in Lysogeny broth, synthetic cystic fibrosis (CF) media 2 (SCFM2) and CF lung media (CFLM) at various growth time points (2, 6, 24 and 72 h). The gene discussed is PROC; the disease is cystic fibrosis.